The present invention relates to the fabrication of the constituent layers of an integrated circuit, both to the removal of material, as by etching, and to the formation of layers, as by deposition or growth. In particular, the invention relates to a laser interferometer system and to laser interferometer techniques for monitoring the complete removal of a layer (etching endpoint detection), for forming or etching layers to a predetermined thickness, and for monitoring the associated time rate of change of the thickness.
Semiconductor integrated circuit (IC) fabrication involves the repetitive application of four basic steps: masking, etching, layer formation (deposition or growth) and doping. Relevant to the present invention, under the continuing impetus to achieve greater device densities and faster device speeds, IC technology has evolved from wet chemcial etching to dry etching using techniques such as physical ion etching (sputtering), plasma chemical etching and reactive ion etching (RIE). Of these, sputtering and reactive ion etching are inherently anisotropic and provide a sharply-defined, steep (high aspect ratio), edge profile which is well suited to meeting the stringent resolution and pattern transfer requirements of VLSI geometries. However, even where the dry etchant has a relatively high selectivity for the etched layer as compared to the material underlying the etched layer, some of the underlying material can be etched away as well. As integrated circuit geometries become smaller and smaller and the minimum feature size approaches one micron and even sub-micron dimensions, scaled structural features such as gate oxides become increasingly susceptible to the slightest overetching. For this reason, most state-of-the-art dry etching systems make some provision for endpoint detection. This monitoring technique involves determining the point at which a layer is etched completely through, and terminating the etching process at that point.
There are available several endpoint detection methods. One approach involves monitoring the composition of the gas adjacent the etched layer for the absence of (or a substantially decreased concentration of) the etched material or its compounds. This condition corresponds to the complete etch-through of the layer.
A second approach, emission endpoint, involves monitoring a characteristic emission wavelength, such as the Al line at 3962 angstroms. The etch is terminated when the intensity of the characteristic emission reduces sharply, indicating a reduction in the amount of the etched material in the discharge.
A third endpoint detection approach uses a laser interferometer. Endpoint detection using laser interferometers is based directly upon depth, unlike processes such as chemical composition monitoring, which are based upon indirect indicia of depth. The resulting combination of accuracy and versatility make this an attractive choice for those applications in which the etched material is transparent to the laser light.
The basis for laser interferometer endpoint detection is shown schematically in FIG. 1, which is a cross-sectional representation of a partially fabricated monolithic integrated circuit 9 taken during anisotropic etching using, for example, reactive ion etching. The layer 10 which is being etched can be formed from any of a number of materials used in IC processing which are transparent to laser light, including dielectric materials such as silicon oxide, organic materials, and silicon in either monocrystalline or polycrystalline (polysilicon) form. The illustrated layer 10 is formed on a substrate 12 such as silicon. During RIE etching, reactive ions are accelerated toward the wafer, as indicated at 14, where they bombard the upper surface 16 of the layer 10. The resulting product is then desorbed, removing material from the upper surface. During endpoint monitoring, a beam 18 of coherent laser light is directed perpendicularly or otherwise onto the upper surface 16. Because of the difference between the refractive indices of the ambient atmosphere and the layer 10, the laser beam is partially reflected at the interface 16 as well as at the interface 17 between the layer 10 and the substrate 12. The interference phenomenon is governed by 2d=N(.lambda./n), where d is the thickness of layer 10, .lambda. is the wavelength of the light and n is the refractive index. For integral values, N=1,2,3 . . . , the reflected light interferes constructively and the reflected intensity is a maximum; for N=1/2, 3/2, 5/2 . . . , the interference is destructive and the reflected intensity is at a minimum. This principle of operation is well known. It bears repeating here, however, that the distance between adjacent maxima, 1/2(.lambda./n), is one-half the effective wavelength of the laser light in the layer 10. This distance provides a convenient basis for determining the thickness of material which has been removed from layer 10 and the time rate of etching. In addition, the characteristic sinusoidal interference pattern of repetitive maxima and minima terminates upon the completion of etching, that is, upon complete removal of the layer 10 to the interface 17. Graphical or electrical monitoring of this change in the interference pattern provides endpoint detection for the purposes of terminating the etch process at the interface 17.
Despite the described versatility, conventional laser interferometer endpoint detection has several important limitations which are addressed by the present invention. Referring to FIG. 2, the first problem relates to the relatively large size of the laser beam 18 relative to certain etch geometries, such as the illustrated contact window 20. Typically, contact windows are very small apertures which are etched through dielectric layers such as the layer 10 using a patterned mask 22. The windows provide vias for making contact to an underlying layer 12, such as a polysilicon gate or conductor, or a substrate contact region. Although FIG. 2 is not to scale, it illustrates somewhat the enormous difference in size between the typical 1-3 micron diameter contact windows 20 and the typical 700 micron diameter laser beam 18. Because of this size difference, etching contact holes in a wafer involves etching only about one percent of the area exposed to the laser beam. The interference signal associated with the etching process is thus very small compared to the background signal and is difficult to detect.
A second problem associated with laser interferometer endpoint detection relates to the topography of the wafer as the IC structure is evolved. Simply stated, the different heights inherent in the IC structure and the angled reflecting surfaces of the IC structure scatter the incident light beam and, again, make it difficult to detect the etching signal.